The present invention relates to the design of ultrasonic transducers used for inspection of solid objects with curved surfaces. Specifically, the inspection utilizes the immersion technique. In this technique, an object is placed in liquid to improve coupling of the ultrasonic wave energy into the part. Ultrasonic energy focused beneath the surface of the inspected object is also addressed, improving sensitivity of inspection in that region.
When an ultrasonic inspection is performed, a transducer is calibrated on a flat surface with the same material to be inspected. A set of inspection parameters, such as gain, operating frequency and water-path, are set and calibrated to the flat-top block. The inspection parameters are used to inspect production hardware. In many cases, the same parameters are used to inspect through curved entry surfaces.
Passing the sound beam through a convex surface decreases the effective sensitivity. However, for a concave surface, the sensitivity of the sound beam increases and then starts to sharply decrease after a certain depth. The effect of a curved surface on inspection sensitivity is very complex. It is difficult to compensate for curved surface effect and keep the same sensitivities as for the flat entry surface.
Many governing agencies and product specifications require that attention be paid to entry surface effects. It is also often required that a curved specimen be used to qualify the inspection process. However, procedure time and cost. and degree of surface curvature variations within a single region make compliance difficult.
It would be desirable then to have a transducer capable of inspecting a part with any curved direction of entry surface.